Cows in Grand Canyon

I was reading New Scientist and flipped through to the Last Word column to find this question: How long would it take an average cow to fill the Grand Canyon with milk?

Bob from New York city answers thus: It all depends upon the size of the tanker truck the cow chooses to drive, the time it would take to drive from the milk distribution point, the inflow and outflow of the tanker truck, the ability to change the absorption and evaporation rates of the milk, and the ability of said cow to effectively block the exit route of the Colorado river.

Other considerations, of course, would be does the cow work an 8-hour day, does she ever get a day off, or does she work 24/7?